The present invention relates to improvements in the field of optical waveguides. More particularly, the invention is concerned with an improved optical fiber having a preset stimulated backward Brillouin scattering threshold for high power transmission application as well as for distributed sensor application.
The attractiveness of coherent optical fiber transmission systems are their long repeater spacing and large transmission capacity. To utilize the available bandwidth and to increase the system margins, narrow optical spectra and high transmitter powers are required. However, these two requirements are limited by optical nonlinear effects such as stimulated backward Brillouin scattering (hereinafter referred to as SBBS). If the optical power launched into a conventional telecommunication type optical fiber exceeds some critical threshold level P.sub.TH, SBBS will occur. In this process, a significant portion of the optical power travelling through the fiber is reflected and converted into a second light wave, shifted in frequency, travelling backward towards the transmitter, and the power level of the transmitted light cannot be increased above a limit.
As reported by R. G. Smith in Applied Optics, Vol. 11, No. 11, pp. 2489-2494, 1972, SBBS not only limits the maximum transmittable optical power, but also disturbs the optical spectra in low-loss long-distance single mode optical fibers. D. Cotter in Journal of Optical Communications, Vol. 4, No. 1, pp. 10-19, 1983, has also observed that the threshold power level P.sub.TH for conventional optical fibers is about 5 milliwatts. It is thus necessary to operate at input power levels below the threshold of SBBS, and this places a severe limitation on the launch power as well as on the repeater spacing.
The 5 milliwatts threshold power level is at least ten times less than the desired level for telecommunication applications. This gives rises to problems in practical long distance optical fiber transmission applications when the input laser power is increased especially with a monochromatic light source. For this telecommunication transmission line application, optical fibers with high P.sub.TH are therefore desired.
However, in the distributed sensor application, the power reflected back to the input end owing to SBBS can be used as a sensing signal for the sensors which are distributed along the length of the optical fiber. For instance, the reflected optical light has a frequency shift which is proportional to the localized acoustic velocity at a specific location along the optical fiber. If there exists a local strain which induces a local stress and thus changes the acoustic velocity, then the frequency of the reflected light will be shifted due to SBBS. Since the frequency shift can be continuously detected, the strain distribution along an optical fiber installation can be monitored. For this distributed sensor application, optical fibers with low P.sub.TH are therefore desired.
For telecommunication transmission line applications, some solutions have been suggested primarily by circumventing the problem, e.g. broadening the spectral line width and distributing the light power over a spectral range. However, no solution has been suggested to increase the P.sub.TH simply because the SBBS phenomena in optical fibers have not been fully understood. Similarly, for the optical fiber distributed sensor application, no effort has been made to reduce the P.sub.TH.